Stepping motors such as that in U.S. Pat. No. 2,931,929, assigned to the assignee of the present invention, are operated by a pulse-to-step motor control circuit. Upon each change of energization of the windings, the motor advances a step. Sustained energization of at least one phase winding is necessary at stand-still to maintain torque.
Due to their inductance, the windings of a stepping motor tend to draw maximum current at stand-still and at low stepping rates, and declining current at increasing speeds. The current supplied to the windings from a high voltage d-c supply can be chopped in control circuits that act, generally, to maintain a desired current level in the windings over a wide range of stepping frequencies, thus improving efficiency and avoiding overheating.
Stepping motors are subject to loss of synchronism at various stepping rates in the broad range of motor speeds, involving loss of torque and the possibility of stalling. This loss of synchronism has been identified with resonance of different types. One type of oscillation occurs at a definite natural resonance frequency of less than 200 steps/sec. for most stepping motors and at sub-harmonics of the natural resonance frequency. There is a discrete integral relationship between the stepping frequency and these resonances which occur at low stepping frequencies. In most practical situations these resonances do not critically limit the performance of a stepping motor system since most motor-and-load combinations can be started instantly at stepping rates well above their natural resonance frequencies. The present invention is not concerned with those low-frequency resonances.
The phenomenon of "mid-frequency resonance" or "mid-range resonance" occurs at a band of stepping rates that are often of prime importance in the motor's application. The motor, its drive and its load tend to lapse into parametric oscillation, which has no integral or rational relationship to the stepping frequency. Instability due to parametric oscillation also tends to develop at bands of higher stepping frequencies. At each such resonance band, the torque may drop seriously and the motor may stall. If the motor is intended to operate in a range of stepping frequencies above a band of instability, some type of acceleration scheme is necessary to drive the motor quickly through such a band of stepping frequencies.
An active stabilizer in U.S. Pat. No. 4,081,736 (assigned to the assignee hereof) counteracts the tendency of a motor and its drive to oscillate. There is still a marked decline of torque in the mid-frequency resonance range. While the active stabilizer in that patent is disclosed in a non-chopping drive, it is also effective in drives of the type in U.S. Pat. No. 4,127,801 that include chopping.
Another useful technique for reducing mid-frequency instability is described in U.S. Pat. No. 3,684,934, assigned to the assignee hereof. In that patent, a small capacitor and diodes are used to draw off and store fly-back or discharge energy of the windings when energizing current is switched off, and a choke allows slow return of the stored energy to the power supply. The circuit is partially effective in avoiding torque reduction due to instability. Unfortunately, that technique does not lend itself readily for use with chopping type drives.
Stepping motors are subject to the further problem of oscillation being triggered both when a new rotational speed is to commence abruptly and when an abrupt change of load occurs. The motor speed hunts above and below the desired speed, gradually settling down to a more-or-less constant speed. The problem of oscillation induced by such abrupt changes has been met in special cases by fluid damping and to some extent by special drive circuits and judicious control over pulse timing. There seems to have been no recognition that positive damping is needed in motor drive circuits to cause rapid settling-down of the motor speed when an abrupt change of speed or load occurs, nor has there been any broad-spectrum solution to the problem of oscillations occurring at intermediate and high motor speeds.